> Thus it seems reasonable to assume that the
> short-period comets were previously long-period comets.

In my mind, the long-period comets offer the strongest evidence of an old
universe. Radioactive decay rates are known to vary, geologic erosion
and deposition rates vary widely, and weathering could be the result of
unknown variables. Long-period comets, entering the solar system with a
velocity near the escape-velocity of the solar system, are similar to a
clock pendulum. As I recall, their period is maybe several hundred
thousand years. All comets are gravitationally bound to the solar
system, suggesting that they originated from the break-up (explosion) of
a planet.